The present invention relates generally to the mechanical measurements of surfaces and, more specifically, to measurement of waviness in product surfaces, such as sheet metal and window glass.
In manufacturing external surface components of an automotive vehicle, controlling the quality of the surface finish is an important consideration. Without regard to the acceptability of the function of a component itself, a component that displays an external surface that is aesthetically unpleasing is often viewed by consumers as a product lacking in quality. Two criteria for judging the aesthetic acceptability of a sheet metal panel relate to the absence of roughness and waviness in the panel. Roughness may be defined for convenience here as microscopic changes in the slope of the surface. "Orange peel" conditions in the painted surface are one example of an unacceptable rough surface on a sheet metal panel. A depression or "streamer" formed in a sheet metal panel during a drawing operation during the stamping of the panel, on the other hand, results in larger macroscopic changes in the slope of the surface defined by that operation. This is waviness. Any surface variation that becomes visible to a consumer when the surface waves are sufficient to cause discontinuities in the reflection of light is an unacceptable level of waviness.
While techniques for measuring roughness are relatively well-developed, the measurement of waviness remains a largely subjective practice dependent at least in part on the visual perception of a wide variety of inspectors. Certain direct measurement techniques are available, however. Among them, the use of expensive profilometers to provide a magnified plot on scaled paper of the profile of a surface. The device is used in conjunction with a reference of the intended design curve of the part itself. Computer aided design data or blueprint information must be compared to the readings of the profilometer and charted for deviations. This is a slow and time-consuming process that cannot effectively be used with great frequency in a high volume production environment such as is encountered in the automotive industry. The recording of a sufficient number of profile data points to define the part's shape for comparison with the intended design also require subjective evaluation of the comparison, tending to dilute its effectiveness. The method utilizing profilometer measurements for evaluating sheet metal panel waviness suffers from the additional defect that it inherently measures absolute values from some fixed base line that is calibrated to the shape of the intended design. Each individual deviation from that base line shape definition must be individually analyzed to determine what effect the deviations have on the aesthetic perception of waviness.
The prior art teaches the use of another mechanism for determining waviness that suffers from the last mentioned defect in that it works about a fixed reference in U.S. Pat. No. 3,470,739 to Takafuji et al.